Atmospheric analysis reveals an exoplanet that’s as alien as can be

HR 8799c is big, but not so big that it's a dwarf.

Infrared image of the HR 8799 system, as seen by the Keck telescope. A new observation has determined the object marked "c" is indeed a planet, but one with unexpected chemical properties.

RC-HIA/C. Marois/Keck Observatory

The star system HR 8799 is a sort of Solar System on steroids: a beefier star, four possible planets that are much bigger than Jupiter, and signs of asteroids and cometary bodies, all spread over a bigger region. Additionally, the whole system is younger and hotter, making it one of only a few cases where astronomers can image the planets themselves. However, HR 8799 is very different from our Solar System, as astronomers are realizing thanks to two detailed studies released this week.

The first study was an overview of the four exoplanet candidates, covered by John Timmer. The second set of observations focused on one of the four planet candidates, HR 8799c. Quinn Konopacky, Travis Barman, Bruce Macintosh, and Christian Marois performed a detailed spectral analysis of the atmosphere of the possible exoplanet. They compared their findings to the known properties of a brown dwarf and concluded that they don't match—it is indeed a young planet. Chemical differences between HR 8799c and its host star led the researchers to conclude the system likely formed in the same way the Solar System did.

The HR 8799 system was one of the first where direct imaging of the exoplanets was possible; in most cases, the evidence for a planet's presence is indirect. (See the Ars overview of exoplanet science for more.) This serendipity is possible for two major reasons: the system is very young, and the planet candidates orbit far from their host star.

The young age means the bodies orbiting the system still retain heat from their formation and so are glowing in the infrared; older planets emit much less light. That makes it possible to image these planets at these wavelengths. (We mostly image planets in the Solar System using reflected sunlight, but that's not a viable detection strategy at these distances). A large planet-star separation means that the star's light doesn't overwhelm the planets' warm glow. Astronomers are also assisted by HR 8799's relative closeness to us—it's only about 130 light-years away.

However, the brightness of the exoplanet candidates also obscures their identity. They are all much larger than Jupiter—each is more than 5 times Jupiter's mass, and the largest could be 35 times greater. That, combined with their large infrared emission, could mean that they are not planets but brown dwarfs: star-like objects with insufficient mass to engage in hydrogen fusion. Since brown dwarfs can overlap in size and mass with the largest planets, we haven't been certain that the objects observed in the HR 8799 system are planets.

For this reason, the two recent studies aimed at measuring the chemistry of these bodies using their spectral emissions. The Palomar study described yesterday provided a broad, big-picture view of the whole HR 8799 system. By contrast, the second study used one of the 10-meter Keck telescopes for a focused, in-depth view of one object: HR 8799c, the second-farthest out of the four.

The researchers measured relatively high levels of carbon monoxide (CO) and water (H2O, just in case you forgot the formula), which were present at levels well above the abundance measured in the spectrum of the host star. According to the researchers, this difference in chemical composition indicated that the planet likely formed via "core accretion"— the gradual, bottom-up accumulation of materials to make a planet—rather than a top-down fragmentation of the disk surrounding the newborn star. The original disk in this scenario would have contained a lot of ice fragments, which merged to make a world relatively high in water content.

In many respects, HR 8799c seemed to have properties between brown dwarfs and other exoplanets, but the chemical and gravitational analyses pushed the object more toward the planet side. In particular, the size and chemistry of HR 8799c placed its surface gravity lower than expected for a brown dwarf, especially when considered with the estimated age of the star system. While this analysis says nothing about whether the other bodies in the system are planets, it does provide further hints about the way the system formed.

One final surprise was the lack of methane (CH4) in HR 8799c's atmosphere. Methane is a chemical component present in all the Jupiter-like planets in our Solar System. The authors argued that this could be due to vigorous mixing of the atmosphere, which is expected because the exoplanet has higher temperatures and pressures than seen on Jupiter or Neptune. This mixing could enable reactions that limit methane formation. Since the HR 8799 system is much younger than the Solar System—roughly 30 million years compared with 4.5 billion years—it's uncertain how much this chemical balance may change over time.

These new observations of HR 8799 could help reveal a lot about planet formation in general. In particular, the core-accretion model is widely thought to be the path the Solar System followed, but that may not be the case in other exosolar systems. Measuring the atmospheric composition of a young, large, hot exoplanet such as HR 8799c is a significant step in learning about the possible ways planets form, and it could help us understand much about the assumptions—correct and incorrect—that go into our models.

32 Reader Comments

This is so awesome to see! Just out of curiosity, but do we know if these objects have moons? The article mentioned that asteroids and comets were detected - but didn't mention moons. I only ask cause based on the size of those planets, I'm thinking my little frame might not last (and thus my great-great-great-great grand kids little frames too), but if there were moons... and those moons were full of water and O2, and close to nice big warm planets... well, we might just have ourselves a nice little spot to call home in about a million or three years!

This is so awesome to see! Just out of curiosity, but do we know if these objects have moons? The article mentioned that asteroids and comets were detected - but didn't mention moons. I only ask cause based on the size of those planets, I'm thinking my little frame might not last (and thus my great-great-great-great grand kids little frames too), but if there were moons... and those moons were full of water and O2, and close to nice big warm planets... well, we might just have ourselves a nice little spot to call home in about a million or three years!

My first thought as well. Probably moons and battle stations are too small to be seen at this point, but we can always hope...

There was a question in the last article's thread about the difference between a "brown dwarf" and a "big-ass planet." The answer given was that a brown dwarf can fuse deuterium but can't sustain a fusion reaction, but this article seems to give more credence to the formation process rather than the properties. So what do you call a mass that forms via core accretion that does eventually initiate fusion?

This is so awesome to see! Just out of curiosity, but do we know if these objects have moons? The article mentioned that asteroids and comets were detected - but didn't mention moons. I only ask cause based on the size of those planets, I'm thinking my little frame might not last (and thus my great-great-great-great grand kids little frames too), but if there were moons... and those moons were full of water and O2, and close to nice big warm planets... well, we might just have ourselves a nice little spot to call home in about a million or three years!

My first thought as well. Probably moons and battle stations are too small to be seen at this point, but we can always hope...

If the moon's massive enough relative to the planet, we should see the planet speed up and slow down ever so slightly as the moon swings ahead and behind it during the planet's orbit around the star. This requires a LOT of imaging, and very precise measurements of the planet's position, but should be possible for this system.

There was a question in the last article's thread about the difference between a "brown dwarf" and a "big-ass planet." The answer given was that a brown dwarf can fuse deuterium but can't sustain a fusion reaction, but this article seems to give more credence to the formation process rather than the properties.

A brown dwarf can sustain fusion, but only deuterium fusion. It cannot sustain fusion of any other element/isotope, and the deuterium gets used up in a few million years.

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So what do you call a mass that forms via core accretion that does eventually initiate fusion?

If you mean "initiates sustained fusion of elements besides deuterium", then the answer is "a star".

I remember having this newspaper clipping hanging up on my bedroom wall when I was a kid in like 1993 that was about the potential discovery of an exoplanet. I was so fascinated with that thought. I didn't think I'd be hearing a growing stream of reports about newly discovered planets, let alone seeing images of them orbiting their stars.

There was a question in the last article's thread about the difference between a "brown dwarf" and a "big-ass planet." The answer given was that a brown dwarf can fuse deuterium but can't sustain a fusion reaction, but this article seems to give more credence to the formation process rather than the properties.

A brown dwarf can sustain fusion, but only deuterium fusion. It cannot sustain fusion of any other element/isotope, and the deuterium gets used up in a few million years.

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So what do you call a mass that forms via core accretion that does eventually initiate fusion?

If you mean "initiates sustained fusion of elements besides deuterium", then the answer is "a star".

I know the researchers used that term, but can we just go back to calling them "star systems"? Or do some planetary/protoplanetary systems exist (somehow) without any stars?

We never called them star systems because THEY ARE NOT STAR SYSTEMS. Planetary systems also do not include things like the debris disk, comets, etc. that are clearly present in this one. So, there just isn't a technical term that describes the whole thing. When the field settles on one, we'll start using it.

The rule of thumb is that brown dwarfs are approximately 13x Jupiter's mass or larger. There's a gray area at the dividing line, but Jupiter is well within the gas giant range.

I'm starting to stretch my knowledge, here, so someone correct me if I'm wrong, but I believe the reason for the gray area for the mass cutoff of a brown dwarf is because it is actually dependent on the temperatures/pressures in the core. Mass is only part of the determinant of those temperatures/pressures. If an object accreted quickly, for instance, it would be hotter than one of the same mass that accreted slowly, simply because there had been less time for energy to radiate away during the accretion process. Similarly, if the original dust cloud had been more dense, then gravitational collapse would convert less energy to heat than if the individual collapsing particles had started the process further apart. Obviously, for objects much smaller or much larger than the 13x dividing line mass becomes the overwhelming determining factor.

Interestingly, though, a brown dwarf would not be much larger in diameter than Jupiter. It would just be more dense and much hotter.

I know the researchers used that term, but can we just go back to calling them "star systems"? Or do some planetary/protoplanetary systems exist (somehow) without any stars?

We never called them star systems because THEY ARE NOT STAR SYSTEMS. Planetary systems also do not include things like the debris disk, comets, etc. that are clearly present in this one. So, there just isn't a technical term that describes the whole thing. When the field settles on one, we'll start using it.

Beat me to it! A "Star System" would be a 'System' of stars. I have also seen the incorrect term 'proto-planetary system' tossed around for this particular system on the web. This one falls in to a relatively new and complicated niche. Fascinating Captain!

Care to elaborate? The distinction doesn't really make sense to me. Albeit I don't know astronomical jargon that well.

When you have two or more stars interacting gravitationally, that is a star system whether or not the system has planets. So if you have a system with a dozen planets and two stars, that's a star system. If you have the same dozen planets and only one star, that is not a star system.

That said, this is awesome! Maybe our grandkids will be able to send something to those systems (doubtful, but I can dream.)

Care to elaborate? The distinction doesn't really make sense to me. Albeit I don't know astronomical jargon that well.

Wileee wrote:

Beat me to it! A "Star System" would be a 'System' of stars.

I'm confused too: I thought a "star system" was any collection of objects where a large fraction (or maybe majority?) of the total mass is located within a central star (or stars, for binaries and such). Other objects could still be quite numerous and varied. Whereas a "system of stars" would be called a cluster, or perhaps some other term that specifies a geometric arrangement.

Care to elaborate? The distinction doesn't really make sense to me. Albeit I don't know astronomical jargon that well.

Wileee wrote:

Beat me to it! A "Star System" would be a 'System' of stars.

I'm confused too: I thought a "star system" was any collection of objects where a large fraction (or maybe majority?) of the total mass is located within a central star (or stars, for binaries and such). Other objects could still be quite numerous and varied. Whereas a "system of stars" would be called a cluster, or perhaps some other term that specifies a geometric arrangement.

Nope - any system that has two or more stars orbiting around a shared center of gravity is a star system. Planets optional. This may or may not be a star system, depending on whether any of the bodies end up being brown dwarfs - we don't know yet. It's clearly a planetary system, as there are at least two planets in the four bodies orbiting it. And it also has a debris disk and signs of additional, smaller planets 5AU and closer (i.e., the debris disk goes away there).

I know the researchers used that term, but can we just go back to calling them "star systems"? Or do some planetary/protoplanetary systems exist (somehow) without any stars?

We never called them star systems because THEY ARE NOT STAR SYSTEMS. Planetary systems also do not include things like the debris disk, comets, etc. that are clearly present in this one. So, there just isn't a technical term that describes the whole thing. When the field settles on one, we'll start using it.

Beat me to it! A "Star System" would be a 'System' of stars.

Then is a solar system a system of solars?

The reason astronomers don't call this a star system is not because the term wouldn't make sense. It's only because they already use the term to mean something else.

The researchers measured relatively high levels of carbon monoxide (CO) and water…

Can readers point me to an accessible piece that explains the distribution of elements we see in our solar system? Does it involve some cataclysmic nuclear reaction, resulting in mostly lighter elements, plus some declining fraction as atomic weights rise, and then from which the lighter elements got chased off, leaving Earth such a large share of iron, silicon & whatnot? Are there particular nuclear pathways that would produce what I guess to be somewhat spiky distributions of atomic weights?

Are these processes believed likely similar to the ones behind the putative planets of HR 8799, with of course the spectra for heavier molecules hidden by lighter ones?

Dr. Jay: I honestly didn't know there was a 2-star minimum. As Chuckstar just said, The Solar System contains exactly one Sol, so that makes the similar-sounding term "star system" genuinely confusing. Anyway thanks, and sorry if you already went over this in the last thread (I didn't have time to participate in that one).

Can readers point me to an accessible piece that explains the distribution of elements we see in our solar system?

Wikipedia is not always the most accessible, but it's a decent starting place. First, a list of key reactions in stellar nucleosynthesis outlines how the elements are produced (e.g.the CNO cycle). Generally it takes increasingly massive stars to produce heavier elements. So the matter comprising our sun & planets came from previous, long-gone stars.

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Does it involve some cataclysmic nuclear reaction, resulting in mostly lighter elements, plus some declining fraction as atomic weights rise...

Generally elements up to iron can be made via nuclear fusion. Anything heavier than iron is made via neutron capture (R and S processes in the above link) during supernovas. So not only did the progenitor stars of our solar system die long ago, but some of them were large enough to go supernova and seed the planets (fairly arbitrarily) with trace heavy elements.

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...and then from which the lighter elements got chased off, leaving Earth such a large share of iron, silicon & whatnot?

The researchers measured relatively high levels of carbon monoxide (CO) and water…

Can readers point me to an accessible piece that explains the distribution of elements we see in our solar system? Does it involve some cataclysmic nuclear reaction, resulting in mostly lighter elements, plus some declining fraction as atomic weights rise, and then from which the lighter elements got chased off, leaving Earth such a large share of iron, silicon & whatnot? Are there particular nuclear pathways that would produce what I guess to be somewhat spiky distributions of atomic weights?

Are these processes believed likely similar to the ones behind the putative planets of HR 8799, with of course the spectra for heavier molecules hidden by lighter ones?

Summary: A solar/exosolar system starts out as a big dust cloud with some net rotation to it. Friction causes the cloud to settle down into a spinning disk of dust. That friction also heats everything up such that the dust actually vaporizes. Then there's a complicated process where different materials condense in different places along the disk. Metals can condense closer to the heat of the star. So you get rocky planets close to the star. Water and other volatiles can only condense out at about the distance of Jupiter from the Sun. So at those distances, you get gas giants and icy bodies. That's why there's always been a question about how Earth got it's water.

I know the researchers used that term, but can we just go back to calling them "star systems"? Or do some planetary/protoplanetary systems exist (somehow) without any stars?

We never called them star systems because THEY ARE NOT STAR SYSTEMS. Planetary systems also do not include things like the debris disk, comets, etc. that are clearly present in this one. So, there just isn't a technical term that describes the whole thing. When the field settles on one, we'll start using it.

Beat me to it! A "Star System" would be a 'System' of stars.

Then is a solar system a system of solars?

The reason astronomers don't call this a star system is not because the term wouldn't make sense. It's only because they already use the term to mean something else.

I never said that it didn't make sense. I simply gave the short answer. John gave the longer and more precise answer. I now see why not many post about the astronomy items. Back to avoiding this comments area.

I never said that it didn't make sense. I simply gave the short answer. John gave the longer and more precise answer. I now see why not many post about the astronomy items. Back to avoiding this comments area.

You said

Quote:

A "Star System" would be a 'System' of stars.

The meaning of "would be" in that context generally implies that there could be no other meaning.

I know the researchers used that term, but can we just go back to calling them "star systems"? Or do some planetary/protoplanetary systems exist (somehow) without any stars?

We never called them star systems because THEY ARE NOT STAR SYSTEMS. Planetary systems also do not include things like the debris disk, comets, etc. that are clearly present in this one. So, there just isn't a technical term that describes the whole thing. When the field settles on one, we'll start using it.

Eh, I'd say that the terminology is even less settled than you suggest. Plenty of folks use the term 'planetary system' more inclusively to include small bodies such as comets or planetesimal belts and debris disks. (The small bodies are "minor planets" after all, in the sense of the Minor Planet Center that tracks asteroid orbits!). Extrasolar system or exosolar systems get some traction but I agree they're kind of clunky. Personally I like the use of "solar systems" lowercase as a generic term, but the language purists say that's no good because the Latin word sol refers specifically to our own sun.

I agree that "star system" is no good, because that term is already used for a system comprising multiple stars. We can talk about a "binary star system" or a "quadruple star system" and that implies nothing about whether or not planets are present about those stars.

Care to elaborate? The distinction doesn't really make sense to me. Albeit I don't know astronomical jargon that well.

When you have two or more stars interacting gravitationally, that is a star system whether or not the system has planets. So if you have a system with a dozen planets and two stars, that's a star system. If you have the same dozen planets and only one star, that is not a star system.

So our Solar System is a system of Solars (or Sols)?!? There ain't but one Sol here.

Another Solar System on steroids is the 4x supersized Vega system, with its asteroid and Kuiper belt debris disks analogs. Its gap in between accommodates many potential giants.

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In particular, the core-accretion model is widely thought to be the path the Solar System followed, but that may not be the case in other exosolar systems.

The problem have been that rapid planet formation has made core accretion seem like a tight fit.

But that is indeed what Corot & Kepler seems to indicate, since while terrestrials are equally frequent regardless of star metal content gas giants becomes smaller and rarer when metallicity goes up. Since a metal rich disk disappears faster, the prediction I've seen is that gas giants _are_ squeezed to grow large. That Jupiter and Saturn are unusually large (as a sample out of the distribution) may be because the Jupiter-Saturn Nice model "Grand Tack", traveling the last gasps (or gaps :-) of the disk twice.

At the same time, core accretion seems to be the norm.

Hopefully it is as simple as that brown dwarfs form as stars and have dense initial disks that can form planets and moons, while gas giants forms as planets (accretion) and have residual accretion disks that can form moons.

Then there's a complicated process where different materials condense in different places along the disk. Metals can condense closer to the heat of the star. So you get rocky planets close to the star. Water and other volatiles can only condense out at about the distance of Jupiter from the Sun. So at those distances, you get gas giants and icy bodies. That's why there's always been a question about how Earth got it's water.

Actually our disk is known to have mixed a lot. You get particles in comets that are heated close to the protostar. It is later that the volatiles tend to freeze out according to different "ice lines".

But the idea that there was a question on how Earth got its water comes from two sources what I am aware of.

One source is that it was observed that enstatite asteroids nearly matched Earth composition but was unusually dry. I think the consensus now is that the rare enstatites have nothing to do with Earth formation.

Another source is that crust metal content shows that it has been resupplied by impactors, and that we have a dense impact record on the Moon et cetera. So it was believed that volatiles came from there.

Since D/H hydrogen isotope ratios shows that comets and asteroids have difficulty to be the only water source, mixed models for water that includes some indigenous accretion water gives the best result. (I'm sure I can find the ref, if asked.)

Mostly, the problem seems to be to predict why Earth got so _little_ water.

"We often talk about the Earth as a water-rich planet; in fact, when astronomers refer to Earth-like planets, we generally mean worlds capable of hosting liquid water. However, in comparison to the gas giants, Earth is actually very water poor! The Earth is only 0.023% water by mass, while the outer solar system giants are as much as 40% water. [And asteroids come in between, so have too much water too.]

The explanation usually invoked to explain this situation is the concept of the “ice line”."

"There’s just one problem: protoplanetary disk models (the mathematical theory and computer simulations behind planet formation) indicate that, as accretion slows and the disk cools, the ice line should migrate inwards – and end up well within the Earth’s orbit! According to the theory as it stands, the Earth should be water-rich and thus much bigger than actually is!"

[Addressing the assumption of magnetic-rotational instability as responsible for disk turbulence, a recent paper finds:]

"This is very exciting – after t~1 million years, there is a growing ice-free region right around the Earth’s orbit! This resolves the discrepancy of previous models, and provides ample time for an ice-free Earth to evolve. Further work will be necessary to validate this model. If it proves consistent, then we may have reconciled planet formation theory with the water-poor Earth: Regions of low turbulence in the protoplanetary disk allow formation of water-poor terrestrial planets."

Best of all, it should be a generic model, predicting that the water poor worlds with oceans and continents that we know and love are frequent. (As opposed to mostly or fully inhabitable terrestrials drowned under many hundreds of km's of water and pressure produced diamond crust.)